AAEP Researchers Discuss Lyme Disease in Horses

It is hard to get a roomful of people to agree on anything, but just about every veterinarian attending the Lyme disease table topic session at the 2013 American Association of Equine Practitioners convention, held Dec. 7-11 in Nashville, Tenn., agreed that more research is needed on this disease in horses. We do know that Lyme disease is an increasingly recognized problem in people living in areas where the causative organism, Borrelia burgdorferi, is endemic. These areas include the northeast and north-central United States. We also know that Borrelia is capable of infecting horses. Despite only a handful of published case reports describing clinical signs in infected horses, many practitioners in endemic areas are convinced that they see cases of Lyme disease in horses. The most common signs in these cases are behavior changes, lethargy or poor attitude, and change in gait/lameness. Uveitis (inflammation within the eye) and neuroborreliosis (neurologic disease due to Lyme disease) are occasionally seen as well.

So if a horse is showing possible signs of Lyme disease, how can it be diagnosed? Several blood tests are available that will identify antibodies against Borrelia. The presence of antibodies (in an unvaccinated horse) indicates that the horse has been infected with Borrelia at some point in its life. The pattern of antibody production might help to ascertain whether infection is acute or chronic. However, more research is needed regarding patterns of antibody production in infected horses. Additionally, many horses infected with Borrelia never show signs of clinical disease, and these horses are what make diagnosing Lyme disease really challenging. Due to the number of infected but asymptomatic horses, veterinarians cannot make a diagnosis of Lyme disease in the horse using only a blood test. The diagnosis should be based on a combination of clinical signs of disease, evidence of exposure to the organism, and ruling out other potential problems.

Once a veterinarian diagnoses Lyme disease, how should he or she treat it? Most veterinarians recommend using a tetracycline-type drug, such as oxytetracycline, doxycycline, or minocycline. Some practitioners recommend intravenous treatment, often placing a long-term catheter, whereas other veterinarians prefer oral medications. It is unclear what the “best” treatment protocol is—again, more research is needed.

In areas where Lyme disease seems to be “everywhere,” how should owners protect their horses? The veterinarians in attendance discussed preventive strategies at length, with most using vaccination as a primary control measure rather than applying topical tick-preventive medications. Although there are no Lyme vaccines labeled for use in horses at the current time, there are several vaccines for dogs that have been used in horses in an extra-label fashion. Most veterinarians did not report any adverse reactions to the canine vaccines, and many observed that the vaccinated horses responded appropriately to the vaccine by producing antibodies. Because different vaccines might induce different patterns of antibody production, it is important to record the name and manufacturer of the vaccine as well as the administration date. This information might help in the interpretation of future blood test results.

This session was moderated by Amy Johnson, DVM, Dipl. ACVIM, assistant professor of large animal medicine and neurology at the University of Pennsylvania's New Bolton Center, and Amy Grice, VMD, of Rhinebeck Equine LLP.

About the Author

Amy Johnson, DVM, Dipl. ACVIM (LAIM and Neurology), is an assistant professor of large animal medicine and neurology at the University of Pennsylvania’s School of Veterinary Medicine. Her research interest is in large animal neurology, particularly in regards to diagnostic testing for neurologic conditions such as botulism and equine protozoal myeloencephalitis (EPM).

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